Reconstructing Palaeoarchaean microbial biomes flourishing in the presence of emergent landmasses using trace and rare earth element systematics
Résumé
Palaeoarchaean cherts preserve the most ancient direct traces of life, but this palaeobiological testament is rarely assimilated into ecosystem or biome models. Trace and rare earth element plus yttrium (REE + Y) compositions reliably decode the palaeodepositional settings of these cherts, and thus constrain the environments within which early microbial life flourished. Herein, we present systematic comparisons between bulk inductively coupled plasma mass spectrometry (ICP-MS) of four fossiliferous cherts from the Barberton greenstone belt, South Africa (the 3.472 Ga Middle Marker horizon, 3.45 Ga Hooggenoeg H5c chert, 3.334 Ga Footbridge Chert, and ~3.33 Ga Josefsdal Chert), and in situ laser ablation (LA) ICP-MS transects through microbial laminations therein. Normalised bulk ICP-MS analyses generally exhibit fractionated REE + Y patterns typical of anoxic hydrogenous sedimentation, supporting previous assertions that the Palaeoarchaean habitable realm was a hydrothermally influenced ocean. Suppressed La, Eu and Y anomalies, together with supra-chondritic Y/Ho ratios, however, indicate restriction from the open ocean and influences from non-marine waters. In situ LA ICP-MS transects through fossiliferous layers yield flat, light REE-enriched REE + Y patterns and chondritic Y/Ho ratios indicating major contributions from terrigenous, riverine fluids, i.e. continental weathering. Resurgences of marine chemistry (increased Y/Ho ratios, La and Y anomalies) occur within microbial laminations themselves. Combined, these results evidence the presence of emergent, volcanic landmasses in the Palaeoarchaean, and highlight the importance of epicontinental basins atop these landmasses as loci for microbial biomes up to 250 Ma before large-scale terrestrial ecosystems. Increased riverine weathering of mafic-felsic continental material, together with periodic seawater recharge into these basins, generated disequilibrium redox conditions under which microbial life flourished. Emergent landmasses may thus have catalysed the flourishing of widespread productive photosynthetic biomes. Charting the relative dominance of biomes through time could illuminate microbial evolutionary trajectories through the lens of environmental reconstruction. Furthermore, we advocate the use of correlated bulk and in situ geochemical approaches in reconstructing ancient environments, since signals relating to small-scale palaeoenvironmental fluctuation can evidently be masked by bulk rock chemistry.